Method for providing hermetic electrical feedthrough

ABSTRACT

This invention provides methods for the processing of platinum metallized high temperature co-fired ceramic (HTCC) components with minimum deleterious reactions between platinum and the glass constituents of the ceramic-glass body. The process comprises co-firing a multilayer laminate green ceramic-glass body with via structures filled with a platinum powder-based material in a reducing atmosphere with a specified level of oxygen partial pressure. The oxygen partial pressure should be maintained above a minimum threshold value for a given temperature level.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional application of U.S. patentapplication Ser. No. 12/473,935, filed May 28, 2009, for Method forProviding Hermetic Electrical Feedthrough, which claims priority to U.S.provisional patent application 61/056,765, for Method for ProvidingHermetic Electrical Feedthrough, filed May 28, 2008. This application isrelated to U.S. Provisional Patent Application Ser. No. 60/946,086,filed Jun. 25, 2007 for Method and Apparatus for Providing HermeticElectrical Feedthrough; U.S. patent application Ser. No. 11/875,198,filed Oct. 19, 2007, for Method for Providing Hermetic ElectricalFeedthrough; U.S. patent application Ser. No. 09/823,464, filed Mar. 30,2001 for Method and Apparatus for Providing Hermetic ElectricalFeedthrough, now U.S. Pat. No. 7,480,988, the disclosures of which areincorporated herein by reference.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The present disclosure was made with support from the United StatesGovernment under Grant number R24EY12893-01, awarded by the NationalInstitutes of Health. The United States Government has certain rights inthe invention.

FIELD

The present disclosure relates generally to a method and apparatus forproviding electrical feedthroughs, and more particularly to a method andapparatus suitable for forming hermetic electrical feedthroughs througha ceramic sheet.

BACKGROUND

Various approaches are described in the literature for fabricatinghermetically sealed electrical circuit housings suitable for extendedoperation in corrosive environments, e.g., in medical devices implantedin a patient's body. For such applications, a housing must be formed ofbiocompatible and electrochemically stable materials and typically mustinclude a wall containing multiple hermetic electrical feedthroughs. Ahermetic electrical feedthrough is comprised of electrically conductivematerial which extends through and is hermetically sealed in the wallmaterial.

One known approach uses an assembled pin feedthrough consisting of aconductive pin that is bonded chemically at its perimeter throughbrazing or the use of oxides, and/or welded, and/or mechanically bondedthrough compression to a ceramic body. Typically, gold is used as abraze material that wets the feedthrough pin and the ceramic bodyresulting in a hermetic seal. Wetting to the ceramic body requires adeposited layer of metal such as titanium. This layer acts additionallyas a diffusion barrier for the gold.

Other alternative feedthrough approaches use a metal tube co-fired witha green ceramic sheet. The hermeticity of the metal/ceramic interface isachieved by a compression seal formed by material shrinkage when theassembly is fired and then allowed to cool. The use of a tube inherentlylimits the smallest possible feedthrough to the smallest availabletubing. Acceptable results have been reported only when using tubeshaving a diameter >40 mils in ceramic substrates at least 70 mils thick.

High temperature co-fired ceramics (HTCCs) are typically constructedwith tungsten-based metallization fired in a reducing atmosphere. Whenthe tungsten is replaced with platinum, particularly in filled vias, thefiring process is complicated by the undesired interaction of platinumwith the ceramic-glass system. A platinum system has a greaterthermodynamic tendency than does tungsten towards reduction of silicaresulting in the volatilization of silicon monoxide and the formation oflow melting temperature platinum silicides. These reactions respectivelylead to devitrification of glass and abnormal microstructure evolutionin liquid phase sintering ultimately producing the following functionaldefects: loss of via hermeticity, loss of electrical continuity, andloss of high temperature stability.

Additionally, since platinum is more resistant to oxidation than istungsten, a platinum-based system may be fired in an atmosphere thatwould be considered oxidizing (i.e. air). However, in an oxidizingenvironment, the formation of volatile platinum oxides would be possibleat high temperatures. This would lead to the following functionaldefects: loss of hermeticity and loss of high temperature stability.

SUMMARY

This invention provides methods for processing of platinum metallizedhigh temperature co-fired ceramic (HTCC) components with minimumdeleterious reactions between platinum and the glass constituents of theceramic-glass body. The process comprises co-firing a multilayerlaminate green ceramic-glass body with via structures filled with aplatinum powder-based material in a reducing atmosphere with a specifiedlevel of oxygen partial pressure. The oxygen partial pressure should bemaintained above a minimum threshold value for a given temperaturelevel.

The initiation of the undesired effects mentioned above can be primarilytraced to the reduction of silicon dioxide. Though there are additionallikely intermediary products and reactants, the following chemicalequation summarizes the overall reaction: SiO₂→Si+O₂

As suggested in the problem statement, replacement of tungsten byplatinum in HTCC increases the thermodynamic tendency of this reductionreaction. The presence of platinum under typical dry reducing firingconditions in effect catalyzes the above reduction due to its highaffinity for interaction with silicon. However, it is at leastconceptually plausible that an increase in the amount of availableoxygen would drive the system to equilibrate itself towards the stableoxide. Indeed a more rigorous analysis of the equation of reactionequilibrium shows that at a given temperature the Gibb's free energychange is shifted increasingly positive with increasing oxygen partialpressure (i.e. partial molar free energy). Additionally, thethermodynamic activity coefficient for silicon dioxide in molten glassaffects the Gibb's free energy change inversely. It follows thatdecreasing this activity coefficient by appropriate choice of glassmaterials would also shift the Gibb's free energy change in the positivedirection.

Therefore the desired manipulation of the Gibb's free energy state ofthe system may be achieved in the following ways:

-   -   1. Control of oxygen partial pressure in the firing atmosphere        to exceed the threshold required for equilibrium at a given        temperature. This can be with a gas mixture of the group        consisting of CO₂/CO, CO₂/NH₃, CO₂/H₂, H₂O/H₂, H₂O/NH₃, H₂O/CO,        Nitrogen, Argon, and vacuum (partial pressure of oxygen 10⁻³⁸        atm to 10⁻³ atm).    -   2. Choice of glass constituents such that the thermodynamic        activity of silicon dioxide in the glass is less than a maximum        at which the Gibb's free energy change for the system is zero.

This invention additionally provides methods for processing of platinummetallized high temperature co-fired ceramic (HTCC) components withminimum deleterious formation of volatile platinum oxide products.

The initiation of the undesired effects mentioned above can be traced tothe formation of a volatile platinum oxide. The following relationshiprepresents the thermodynamic equilibrium between the metal, oxygen, andgaseous oxide:

$\left. {{xPt} + {\frac{1}{2}{yO}_{2}}}\rightleftarrows{{Pt}_{x}O_{y}} \right.$

The desired manipulation of the Gibb's free energy state of the systemmay be achieved in the following ways:

-   -   1. It is preferred to minimize the partial pressure of oxygen as        low as possible to limit the thermodynamic tendency for gaseous        platinum oxide formation (i.e. Manipulate the sign of the Gibbs        free energy change increasingly positive). An oxygen partial        pressure less than 10⁻³ atm is desired. However, even though        this number is exceeded, it is advantageous to have an oxygen        partial pressure less than in atmospheric air.    -   2. Another approach is to include an additional platinum oxide        gas generating source into the furnace during firing of platinum        via parts. For example, a pre-fired ceramic substrate with        sputtered platinum layer or a ceramic crucible with fine        platinum powder may be used. This would cause additional        platinum oxide gas to be formed in the chamber minimizing the        degradation of the platinum contained in the fabricated part.        The amount and geometric configuration of this ‘sacrificial’        platinum may be tailored such that degradation occurs        preferentially on the ‘sacrificial’ platinum over the component        platinum.

The mechanical system is able to accommodate some deleterious reactionsoccurring when the oxygen partial pressure is outside a theoreticalrange either toward the reducing side or the oxidizing size. The degreeof the deviation will determine the amount of undesired phenomena. Evenmoderate changes in the Gibb's free energy state of the system may yieldmeasureable differences in the fabricated part.

Additionally, it should be mentioned that temperature is also animportant parameter in the firing process. Temperature may also be usedto favorably manipulate the free energy state of the reactions.

Further embodiments are shown in the specification, drawings and claimsof the present application.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a top view of a finished feedthrough assembly inaccordance with the present disclosure comprised of a ceramic sheethaving electrically conductive vias extending therethrough;

FIG. 2 depicts a sectional view taken substantially along the plane 2-2of FIG. 1, showing the electrically conductive vias ends flush with thesurfaces of the ceramic sheet;

FIG. 3 depicts a flow diagram illustrating a possible series of processsteps for fabricating a feedthrough assembly in accordance with thepresent disclosure;

FIGS. 4-6 respectively depict the fabrication stages of a feedthroughassembly in accordance with the process flow illustrated in FIG. 3,wherein FIG. 4A depicts a sectional view of a ceramic sheet;

FIGS. 4B-4C depict via holes being punched in the sheet of FIG. 4A;

FIGS. 4D-4E depict exemplary stencil printing with vacuum pull downprocess;

FIG. 5A depicts paste inserted into the via holes;

FIGS. 5B-C depict exemplary multilayer lamination process;

FIG. 6A shows an exemplary laminated substrate;

FIGS. 6B-C depict lapping/grinding process; and

FIGS. 6D-E depict dicing of the substrate to form multiple feedthroughassemblies.

FIG. 7 is a perspective view of the implanted portion of the preferredvisual prosthesis.

FIG. 8 is a side view of the implanted portion of the preferred visualprosthesis showing the fan tail in more detail.

FIG. 9 is a view of the completed package attached to an electrodearray.

FIG. 10 is a cross-section of the package.

In the following description, like reference numbers are used toidentify like elements. Furthermore, the drawings are intended toillustrate major features of exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of everyimplementation nor relative dimensions of the depicted elements, and arenot drawn to scale.

DETAILED DESCRIPTION

The present disclosure is directed to a method and apparatus suitablefor forming hermetic electrical feedthroughs in a ceramic sheet (orsubstrate) having a possible thickness of ≤40 mils. More particularly,the disclosure is directed to a method and apparatus for forming astructure including a hermetic electrical feedthrough which is bothbiocompatible and electrochemically stable and suitable for implantationin a patient's body.

Electrical feedthroughs in accordance with the present writing areintended to function in corrosive environments, e.g., in medical devicesintended for implantation in a patient's body. In such applications, itis generally critical that the device housing be hermetically sealedwhich, of course, requires that all feedthroughs in the housing wallalso be hermetic. In such applications, it is also generally desirablethat the weight and size of the housing be minimized and that allexposed areas of the housing be biocompatible and electrochemicallystable. Biocompatibility assures that the implanted device has nodeleterious effect on body tissue. Electrochemical stability assuresthat the corrosive environment of the body has no deleterious effect onthe device. Ceramic and platinum materials are often used in implantablemedical devices because they typically exhibit both biocompatibility andelectrochemical stability.

Embodiments constructed in accordance with the present disclosure areable to achieve very high feedthrough density. For example, inapplications where miniaturization is important, the feedthrough pitch,i.e., center-to-center distance between adjacent feedthroughs may befrom 10 mils to 40 mils.

Attention is initially directed to FIGS. 1 and 2 which depict apreferred feedthrough assembly 8 in accordance with the presentdisclosure comprising a thin ceramic sheet 10 of ceramic material havingmultiple electrical feedthroughs 12 extending therethrough terminatingflush with the upper and lower surfaces 14, 16 of sheet 10. The sheet 10typically comprises a wall portion of a housing (not shown) foraccommodating electronic circuitry. The feedthroughs 12 function toelectrically connect devices external to the housing, e.g., adjacent tosurface 14, to electronic circuitry contained within the housing, e.g.,adjacent to surface 16. “Thin ceramic sheet” as used herein refers to asheet having a finished thickness dimension of ≤40 mils, i.e., 1 mm. Theapparatus in accordance with the disclosure is particularly suited foruse in corrosive environments such as in medical devices implanted in apatient's body.

The present disclosure is directed to providing electrical feedthroughsthat are compatible with thin ceramic sheets (or substrates) having afinished thickness of ≤40 mils, and with feedthroughs that are hermetic,biocompatible, and electrochemically stable. In one exemplaryembodiment, the ceramic sheet 10 may be formed of 90% aluminum oxide(AlO₂) and the feedthroughs 12 may have a diameter of ≤20 mils and maybe composed of paste containing, for example, platinum.

Attention is now directed to FIGS. 3-6E which depict the possibleprocess steps for fabricating the finished feedthrough assembly 8illustrated in FIGS. 1 and 2.

Initially, a green ceramic sheet/tape/substrate 20 (FIG. 4A), formed,for example, of >90% aluminum oxide (AlO₂) is selected as represented bystep 21 in FIG. 3. In an exemplary embodiment, the sheet 20 may have athickness of 40 mils or less. “Green ceramic sheet/tape/substrate” asused herein refers to an unfired ceramic sheet, tape or substrate.

Via holes 26 are formed into the sheet 20 as represented by FIGS. 4B-4Cand step 28 in FIG. 3. In an exemplary embodiment, each via hole 26 maybe punched in to the sheet 20 using, for example, programmable punchtool 27. In one exemplary embodiment, a plurality of via holes 26 may bepunched at the same time. It is to be understood that other methods maybe used to form via holes 26. For example, via holes 26 may be formedusing solvent etching, laser ablation, and/or via holes 26 may bedrilled.

Step 37 of FIG. 3 calls for selecting a conductive thickfilm paste 17 tofill in via holes 26 depicted in FIG. 4C. “Thickfilm paste” as usedherein refers to a material containing inorganic particles dispersed ina vehicle comprising an organic resin and a solvent. Types of differentpastes are disclosed in U.S. Pat. No. 5,601,638, the disclosure of whichis incorporated herein by reference.

In one exemplary embodiment, a stencil printing with vacuum pull downprocess may be used to fill via holes 26 with the conductive paste 17 asrepresented by FIGS. 4D-4E and step 39 in FIG. 3. During the stencilprinting with vacuum pull down process, the sheet 20 may be sandwichedbetween a stencil layer 19 and a vacuum base 80. As a squeegee 18 rollsthe conductive paste 17 across the stencil layer 19, a vacuum chuck 81of the vacuum base 80 pulls the conductive paste 17 through holes 82 ofthe stencil layer 19 and into the via holes 26 as shown in FIGS. 4D-4E.

Step 40 of FIG. 3 calls for determining if additional green ceramicsheet/tape/substrates with paste filled via holes are required. Ifadditional green ceramic sheet/tape/substrates with paste filled viaholes are required (“Yes” in step 40), steps 21, 28, 37 and 39 arerepeated. If additional green ceramic sheet/tape/substrates with pastefilled via holes are not required (“No” in step 40), step 41 of FIG. 3is performed.

Upon completion of the stencil printing with vacuum pull down processand step 40, the sheet 20 with via holes 26 filled with conductive paste17 shown in FIG. 5A may go through a multilayer lamination process asrepresented by FIGS. 5B-5C and step 41 in FIG. 3.

In the multilayer lamination process, the sheet 20 of FIG. 5A may belaminated with, for example, sheets 91 and 92 as shown in FIG. 5B. Thesheets 91 and 92 may contain conductive paste filled vias 26 that aresimilar to the conductive paste filled vias 26 of the sheet 20, and thesheets 91 and 92 may be formed using steps 21, 28, 37 and 39 of FIG. 3as described above.

During the multilayer lamination process, a) the sheets 20, 91 and 92are stacked on top of each other with conductive paste filled vias 26 ofeach sheet being aligned on top of each other; b) stacked sheets 20, 91and 92 are sandwiched between two unpunched green ceramicsheets/tapes/substrates 95 and 96; and c) the sheets 20, 91 and 92 andthe sheets 95 and 96 are laminated together using a heatpress 98 tocreate a laminated substrate 100 shown in FIG. 6A.

Although FIGS. 5B and 5C laminate three sheets 20, 91 and 92 withconductive paste filled vias 26, one skilled in the art can appreciatethat this disclosure is not limited to three sheets and that a singlesheet 20 with conductive paste filled vias may be laminated togetherwith the sheets 95 and 96 without the additional sheets 91 and 92.Although FIGS. 5B and 5C laminate three sheets 20, 91 and 92 withconductive paste filled vias 26, one skilled in the art can appreciatethat this disclosure is not limited to three sheets and that additionalsheets with conductive paste filled vias may also be laminated togetherwith sheets 20, 91 and 92.

Step 44 of FIG. 3 calls for the laminated substrate 100 to be fired.Firing of the laminated substrate 100 encompasses different aspects offorming bonds in ceramic (evaporation, binder burnout, sintering, etc.).The unpunched ceramic layers 95 and 96 of the laminated substrate 100help to constrain the conductive paste within via holes 26 and allow forcompression during the firing step 44. The unpunched ceramic layers 95and 96 of the laminated substrate 100 also help to isolate theconductive paste filled vias 26 from the firing atmosphere during thestep 44 which may be the key to hermetic and low resistance paste filledvias 26. An exemplary firing schedule includes ramping the laminatedsubstrate 100 of FIG. 6A up to 600° C. at a rate of 1° C./minute, thenramping up to 1600° C. at a rate at 5° C./minute, followed by a one hourdwell and then a cool-to-room-temperature interval.

This invention provides methods for processing of platinum metallizedhigh temperature co-fired ceramic (HTCC) components with minimumdeleterious reactions between platinum and the glass constituents of theceramic-glass body. The process comprises co-firing a multilayerlaminate green ceramic-glass body with via structures filled with aplatinum powder-based material in a reducing atmosphere with a specifiedlevel of oxygen partial pressure. The oxygen partial pressure should bemaintained above a minimum threshold value for a given temperaturelevel.

The initiation of the undesired effects mentioned above can be primarilytraced to the reduction of silicon dioxide. Though there are additionallikely intermediary products and reactants, the following chemicalequation summarizes the overall reaction: SiO₂→Si+O₂.

As suggested in the problem statement, replacement of tungsten byplatinum in HTCC increases the thermodynamic tendency of this reductionreaction. The presence of platinum under typical dry reducing firingconditions in effect catalyzes the above reduction due to its highaffinity for interaction with silicon. However, it is at leastconceptually plausible that an increase in the amount of availableoxygen would drive the system to equilibrate itself towards the stableoxide. Indeed a more rigorous analysis of the equation of reactionequilibrium shows that at a given temperature the Gibb's free energychange is shifted increasingly positive with increasing oxygen partialpressure (i.e. partial molar free energy). Additionally, thethermodynamic activity coefficient for silicon dioxide in molten glassaffects the Gibb's free energy change inversely. It follows thatdecreasing this activity coefficient by an appropriate choice of glassmaterials would also shift the Gibb's free energy change in the positivedirection. Therefore the desired manipulation of the Gibb's free energystate of the system may be achieved in the following ways:

-   -   1. Control of oxygen partial pressure in the firing atmosphere        to exceed the threshold required for equilibrium at a given        temperature. This can be with a gas mixture of the group        consisting of CO₂/CO, CO₂/NH₃, CO₂/H₂, H₂O/H₂, H₂O/NH₃, H₂O/CO,        Nitrogen, Argon, and vacuum (partial pressure of oxygen 10⁻³⁸        atm to 10⁻³ atm).    -   2. Choice of glass constituents such that the thermodynamic        activity of silicon dioxide in the glass is less than a maximum        at which the Gibb's free energy change for the system is zero.

This invention additionally provides methods for processing of platinummetallized high temperature co-fired ceramic (HTCC) components withminimum deleterious formation of volatile platinum oxide products.

The initiation of the undesired effects mentioned above can be traced tothe formation of a volatile platinum oxide. The following relationshiprepresents the thermodynamic equilibrium between the metal, oxygen, andgaseous oxide:

$\left. {{xPt} + {\frac{1}{2}{yO}_{2}}}\rightleftarrows{{Pt}_{x}O_{y}} \right.$

The desired manipulation of the Gibb's free energy state of the systemmay be achieved in the following ways:

-   -   1. It is preferred to minimize the partial pressure of oxygen as        low as possible to limit the thermodynamic tendency for gaseous        platinum oxide formation (i.e. Manipulate the sign of the Gibbs        free energy change increasingly positive). An oxygen partial        pressure less than 10⁻³ atm is desired. However, even though        this number is exceeded, it is advantageous to have an oxygen        partial pressure less than in atmospheric air.    -   2. Another approach is to include an additional platinum oxide        gas generating source into the furnace during firing of platinum        via parts. For example, a pre-fired ceramic substrate with        sputtered platinum layer or a ceramic crucible with fine        platinum powder may be used. This would cause additional        platinum oxide gas to be formed in the chamber minimizing the        degradation of the platinum contained in the fabricated part.        The amount and geometric configuration of this ‘sacrificial’        platinum may be tailored such that degradation occurs        preferentially on the ‘sacrificial’ platinum over the component        platinum.

The mechanical system is able to accommodate some deleterious reactionsoccurring when the oxygen partial pressure is outside a theoreticalrange either toward the reducing side or the oxidizing size. The degreeof the deviation will determine the amount of undesired phenomena. Evenmoderate changes in the Gibb's free energy state of the system may yieldmeasureable differences in the fabricated part.

Additionally, it should be mentioned that temperature is also animportant parameter in the firing process. Temperature may also be usedto favorably manipulate the free energy state of the reactions.

During the firing and subsequent cooling of the firing step 44, theceramic material of the laminated substrate 100 shrinks, therebyshrinking via holes 26 around the paste 17 to form a seal. The finealuminum oxide suspension permits uniform and continuous sealing aroundthe surface of the paste 17. Additionally, at the maximum firingtemperature, e.g., 1600° C., the paste 17 being squeezed by the ceramicexhibits sufficient flow to enable the paste 17 to flow and fill anycrevices in the ceramic. This action produces a hermetic paste/ceramicinterface. Furthermore, the firing step 44 may also cause hermeticitythrough bonding mechanisms like, for example, sintering, glassmelt/wetting, alloying, compounding and/or diffusion solution formation.“Sintering” as used herein is a term used to describe the consolidationof the ceramic material during firing. Consolidation implies that withinthe ceramic material, particles have joined together into an aggregatethat has strength. The term sintering may be used to imply thatshrinkage and densification have occurred; although this commonlyhappens, densification may not always occur. “Sintering” is also amethod for making objects from powder, by heating the material (belowits melting point) until its particles adhere to each other. “Sintering”is traditionally used for manufacturing ceramic objects, and has alsofound uses in such fields as powder metallurgy. “Alloying” as usedherein refers to an alloy that is a homogeneous hybrid of two or moreelements, at least one of which is a metal, and where the resultingmaterial has metallic properties. “Compounding” as used herein refers toa chemical compound that is a substance consisting of two or moreelements chemically-bonded together in a fixed proportion by mass.“Diffusion solution formation” as used herein refers to the net movementof particles from an area of high concentration to an area of lowconcentration. A solid solution is a solid-state solution of one or moresolutes in a solvent. Such a mixture is considered a solution ratherthan a compound when the crystal structure of the solvent remainsunchanged by addition of the solutes, and when the mixture remains in asingle homogeneous phase. Also, the firing step 44 may also causesolidification of the metallized vias 26 and the ceramic material of thelaminated substrate 100 to prevent leaks.

Step 48 of FIG. 3 calls for lapping or grinding the upper and lowersurfaces of the fired laminated substrate 100 to remove materials 50 and51, depicted in FIG. 6B, in order to expose the upper and lower faces ofthe metallized vias 26. The upper and lower surfaces of the firedlaminated substrate 100 may also go through the polishing step 49 sothat the metallized vias 26 are flush with the surrounding ceramicmaterial.

After lapping and/or grinding, the fired laminated substrate 100 may besubjected to a hermeticity test, e.g., frequently a helium (He) leaktest as represented by step 56 in FIG. 3.

In one exemplary embodiment, sheet/substrate 20 may contain severalpatterns 24 a-d of the via holes 26 as shown in FIG. 6D. In thisexemplary embodiment, the fired laminated substrate 100 would containseveral patterns 24 a-d of the metal filled via holes 26 and the firedlaminated substrate 100 would be subjected to a singulation or dicingstep 58 to provide multiple feedthrough assemblies 60A, 60B, 60C, 60Dshown in FIG. 6E.

Although some embodiments described above employ a ceramic sheet of >90%aluminum oxide (AlO₂), alternative embodiments may use other ceramicmaterials, e.g., zirconium. Because the firing temperature of theceramic can be tailored within certain limits, the conductive paste 17may comprise any of the noble metals and/or any of the refractorymetals, for example, platinum, titanium, gold, palladium, tantalum,niobium.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. The term “plurality” includes two or morereferents unless the content clearly dictates otherwise. Unless definedotherwise, all technical and scientific terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art towhich the disclosure pertains.

From the foregoing, it should now be appreciated that electricalfeedthrough assemblies and fabrication methods thereof have beendescribed suitable for use in medical devices intended for implantationin a patient's body. Although a specific structure and fabricationmethod has been described, it is recognized that variations andmodifications will occur to those skilled in the art coming within thespirit and scope of the invention as defined by the appended claims.

Referring to FIGS. 7 through 10, the preferred use for the feedthroughof the present invention is in an implantable hermetic package such asused in a visual prosthesis. An implanted electronic device, such as avisual prosthesis, requires that the electronics be encapsulated in ahermetic biocompatible package that allows for electrical signals topass through the package with a metalized via.

FIG. 7 shows a perspective view of the implanted portion of thepreferred visual prosthesis. A flexible circuit 101 includes a flexiblecircuit electrode array 110 which is mounted by a retinal tack (notshown) or similar means to the epiretinal surface. The flexible circuitelectrode array 110 is electrically coupled by a flexible circuit cable112, which pierces the sclera and is electrically coupled to anelectronics package 114, external to the sclera.

The electronics package 114 is electrically coupled to a secondaryinductive coil 116. Preferably the secondary inductive coil 116 is madefrom wound wire. Alternatively, the secondary inductive coil 116 may bemade from a flexible circuit polymer sandwich with wire traces depositedbetween layers of flexible circuit polymer. The secondary inductive coilreceives power and data from a primary inductive coil 117, which isexternal to the body. The electronics package 114 and secondaryinductive coil 116 are held together by the molded body 118. The moldedbody 118 holds the electronics package 114 and secondary inductive coil116 end to end. The secondary inductive coil 116 is placed around theelectronics package 114 in the molded body 118. The molded body 118holds the secondary inductive coil 116 and electronics package 114 inthe end to end orientation and minimizes the thickness or height abovethe sclera of the entire device. The molded body 118 may also includesuture tabs 120. The molded body 118 narrows to form a strap 122 whichsurrounds the sclera and holds the molded body 118, secondary inductivecoil 116, and electronics package 114 in place. The molded body 118,suture tabs 120 and strap 122 are preferably an integrated unit made ofsilicone elastomer. Silicone elastomer can be formed in a pre-curvedshape to match the curvature of a typical sclera. However, siliconeremains flexible enough to accommodate implantation and to adapt tovariations in the curvature of an individual sclera. The secondaryinductive coil 116 and molded body 118 are preferably oval shaped. Astrap 122 can better support an oval shaped coil. It should be notedthat the entire implant is attached to and supported by the sclera. Aneye moves constantly. The eye moves to scan a scene and also has ajitter motion to improve acuity. Even though such motion is useless inthe blind, it often continues long after a person has lost their sight.By placing the device under the rectus muscles with the electronicspackage in an area of fatty tissue between the rectus muscles, eyemotion does not cause any flexing which might fatigue, and eventuallydamage, the device.

FIG. 8 shows a side view of the implanted portion of the visualprosthesis, in particular emphasizing the fan tail 124. When implantingthe visual prosthesis, it is necessary to pass the strap 122 under theeye muscles to surround the sclera. The secondary inductive coil 16 andmolded body 118 must also follow the strap 122 under the lateral rectusmuscle on the side of the sclera. The implanted portion of the visualprosthesis is very delicate. It is easy to tear the molded body 118 orbreak wires in the secondary inductive coil 116. In order to allow themolded body 118 to slide smoothly under the lateral rectus muscle, themolded body 118 is shaped in the form of a fan tail 124 on the endopposite the electronics package 114. The strap 122 further includes ahook 128 that aids the surgeon in passing the strap under the rectusmuscles.

Referring to FIG. 9, the flexible circuit 101 includes platinumconductors 194 insulated from each other and the external environment bya biocompatible dielectric polymer 196, preferably polyimide. One end ofthe array contains exposed electrode sites that are placed in closeproximity to the retinal surface 10. The other end contains bond pads192 that permit electrical connection to the electronics package 114.The electronics package 114 is attached to the flexible circuit 101using a flip-chip bumping process, and is epoxy underfilled. In theflip-chip bumping process, bumps containing conductive adhesive placedon bond pads 192 and bumps containing conductive adhesive placed on theelectronic package 114 are aligned and melted to build a conductiveconnection between the bond pads 92 and the electronics package 114.Leads 176 for the secondary inductive coil 116 are attached to gold pads178 on the ceramic substrate 160 using thermal compression bonding, andare then covered in epoxy. The electrode array cable 112 is laser weldedto the assembly junction and underfilled with epoxy. The junction of thesecondary inductive coil 116, array 101, and electronics package 114 areencapsulated with a silicone overmold 190 that connects them togethermechanically. When assembled, the hermetic electronics package 114 sitsabout 3 mm away from the end of the secondary inductive coil.

Since the implant device is implanted just under the conjunctiva it ispossible to irritate or even erode through the conjunctiva. Erodingthrough the conjunctiva leaves the body open to infection. We can doseveral things to lessen the likelihood of conjunctiva irritation orerosion. First, it is important to keep the over all thickness of theimplant to a minimum. Even though it is advantageous to mount both theelectronics package 114 and the secondary inductive coil 116 on thelateral side of the sclera, the electronics package 114 is mountedhigher than, but not covering, the secondary inductive coil 116. Inother words, the thickness of the secondary inductive coil 116 andelectronics package should not be cumulative.

It is also advantageous to place protective material between the implantdevice and the conjunctiva. This is particularly important at thescleratomy, where the thin film electrode array cable 112 penetrates thesclera. The thin film electrode array cable 112 must penetrate thesclera through the pars plana, not the retina. The scleratomy is,therefore, the point where the device comes closest to the conjunctiva.The protective material can be provided as a flap attached to theimplant device or a separate piece placed by the surgeon at the time ofimplantation. Further, material over the scleratomy will promote healingand sealing of the scleratomy. Suitable materials include DACRON®,TEFLON®, GORETEX® (ePTFE), TUTOPLAST® (sterilized sclera), MERSILENE®(polyester) or silicone.

Referring to FIG. 10, the package 114 contains a ceramic substrate 160,with metalized vias 165 and thin-film metallization 166. The package 114contains a metal case wall 62 which is connected to the ceramicsubstrate 60 by braze joint 161. On the ceramic substrate 160 anunderfill 169 is applied. On the underfill 169 an integrated circuitchip 164 is positioned. On the integrated circuit chip 64 a ceramichybrid substrate 168 is positioned. On the ceramic hybrid substrate 168passives 170 are placed. Wire bonds 167 are leading from the ceramicsubstrate 60 to the ceramic hybrid substrate 168. A metal lid 184 isconnected to the metal case wall 162 by laser welded joint 163 wherebythe package 114 is sealed.

Accordingly, what has been shown is an improved method of making aneural electrode array and an improved method of stimulating neuraltissue. While the invention has been described by means of specificembodiments and applications thereof, it is understood that numerousmodifications and variations could be made thereto by those skilled inthe art without departing from the spirit and scope of the invention. Itis therefore to be understood that within the scope of the claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. A method of fabricating a hermetic electricalfeedthrough, the method comprising: providing a ceramic sheet, andhaving an upper surface and a lower surface, wherein said sheet includessilicon dioxide; forming at least one via hole in said sheet extendingfrom said upper surface to said lower surface; inserting a conductivethickfilm paste, including platinum, into said via hole; laminating saidsheet with said conductive thick film paste in said via hole between anupper ceramic sheet, and a lower ceramic sheet, to form a laminatedsubstrate wherein said via hole is encased in ceramic; selecting anoxygen controlled environment, between 10⁻³⁸ and 10⁻³ atmospheres ofoxygen partial pressure, by introduction of other gasses selected fromthe group consisting of C0₂/CO, C0₂/NH₃, C0₂/H₂, H₂0/H₂, H₂0/NH₃,H₂0/CO, Nitrogen, or Argon, to balance platinum oxidation and silicondioxide decomposition of said laminated substrate; firing said laminatedsubstrate in said oxygen controlled environment to a temperature tosinter said laminated substrate to form a single sintered structure andcause said conductive thick film paste in said via hole to form ametallized via, including platinum in contact with ceramic, and causesaid laminated substrate to form a fired laminated substrate and ahermetic seal around said metallized via; and removing said upper sheetmaterial and said lower sheet material by lapping or grinding, to exposean upper and a lower surface of said metallized via.
 2. The methodaccording to claim 1, wherein said sheet is formed of material comprisedof at least 90% aluminum oxide.
 3. The method according to claim 1,wherein said fired laminated substrate after said firing and materialremoval steps is less than or equal to 40 mils thick.
 4. The methodaccording to claim 1, wherein said fired laminated substrate after saidfiring and material removal steps is less than 15 mils thick.
 5. Themethod according to claim 1, wherein forming at least one via hole insaid sheet comprises punching said via hole using a punch tool, etchingsaid via hole using a solvent etching, using laser ablation or drillingsaid via hole.
 6. The method according to claim 1, wherein inserting aconductive material into said via hole comprises: disposing said sheetwith said via hole between a stencil layer and a vacuum base, whereinsaid stencil layer includes at least one through hole that is alignedabove said via hole; rolling said conductive material across saidstencil layer; and pulling said conductive material into said via holethrough said hole in said stencil layer with a vacuum created by saidvacuum base.
 7. The method according to claim 1, further comprisinglaminating said sheet with paste filled via holes between said uppersheet and said lower sheet to form said laminated substrate; placingsaid sheet with paste filled via holes together with said upper ceramicsheet and said lower ceramic sheet in a heatpress; and applying heat andpressure by said heatpress until said laminated substrate is formed. 8.The method according to claim 1, wherein said ceramic material comprisesaluminum oxide, zirconium oxide or a mixture thereof.
 9. The methodaccording to claim 1, wherein said fired laminated substrate has athickness of less than 20 mils after removing said upper ceramic sheetand said lower ceramic sheet material.
 10. The method according to claim1, wherein said fired laminated substrate has a thickness of 15-20 milsafter removing said upper ceramic sheet and said lower ceramic sheetmaterial.